1. Field of the Invention
The invention relates to a method for duplex data transmission with quadrature amplitude modulation (QAM) via two conductors using the time division multiplex and frequency division multiplex method of two oppositely transporting channels. The transmission is in a first direction via an xe2x80x9cupperxe2x80x9d channel with a higher frequency band foxc2x1Bo/2 and, in a second direction, via a xe2x80x9clowerxe2x80x9d channel with a lower frequency band fuxc2x1Bu/2, which does not overlap the higher frequency band. Each of the channels transports two digital signals, namely a normal and a quadrature data signal, at a respective symbol frequency fT, which may be identical or different for the two channels. The system further performs at both ends modulation of a carrier signal with the two digital data signals for the purpose of transmission, and sampling of the received modulated carrier signal at a sampling frequency fAo in the case of the upper channel and fAu in the case of the lower channel, respectively. The pulse train resulting from the sampling is subsequently demodulated, which is followed by low-pass filtering and sampling at the symbol frequency of the data signal for the purpose of recovering the two digital data signals used for modulation at the other end. The following frequencies are defined:
fo=center frequency of the two upper channels;
Bo=bandwidth of the upper channels;
fu=center frequency of the lower channels;
Bu=bandwidth of the lower channels;
fT=symbol frequency of the digital data signal;
fAo=sampling frequency of the receiver-end sampling of the upper channel prior to demodulation;
fAu=sampling frequency of the receiver-end sampling of the lower channel prior to demodulation;
and the following inequalities hold true:
fo greater than fu; fAo greater than fT; and fAu greater than fT.
The invention furthermore relates to a demodulator for use in a preferred embodiment of the method according to the invention, having a respective demodulation path for the normal signal and the quadrature signal. The demodulator has a first sampler for sampling at a sampling frequency, a multiplier for multiplication by a cyclic signal, a low-pass filter, and a second sampler for sampling at the symbol frequency.
In the case of duplex data transmission via two-wire lines, the transmission is interfered with to an increasing extent by near-end crosstalk as the speed increases. This is due to the fact that near-end crosstalk attenuation decreases with increasing frequency. In the case of the time division multiplex method with echo compensation, which utilizes the channel bandwidth optimally, from a certain speed the range is limited by the near-end crosstalk. The range can then be increased only by compensating for the near-end crosstalk, which requires considerable technical sophistication that cannot yet be realized. If the time division multiplex method is combined with the frequency division multiplex method, in which the transmission in the two directions takes place in different frequency bands, then although the required bandwidth is increased, the near-end cross-talk can be eliminated by suitable selective filtering. A precondition in this case is that all the signals transmitted in the same direction are transmitted in a multi-pair cable in the same frequency band. From speeds of a few Mbits and above, a greater range can be achieved with the frequency division multiplex method than with the time division multiplex method with echo compensation.
As the transmission method, the quadrature amplitude modulation method is taken as a basis. This enables a plurality of bits to be transmitted per symbol by means, for example, of quaternary or even higher encryption (cf. the textbook by K. D. Kammeyer xe2x80x9cNachrichtenubertragungxe2x80x9d [Tele-communications], Stuttgart 1996, p. 372, FIGS. 11.1.2). According to the frequency division multiplex method, transmission in one direction takes place in a different frequency band from transmission in the other direction, in which case, in order to be able to eliminate the near-end crosstalk by selective filtering, the carrier frequencies for the two transmission directions are chosen in such a way that the two bands do not overlap.
At the receiver end, the received modulated signal has to be filtered out of the composite signal on the line and sampled at a suitable sampling frequency. The choice of sampling frequency has a very great influence on the realization complexity. The lower it can be chosen to be, the lower the complexity is with regard to processing speed and number of coefficients in the case of the digital filtering. The sampling frequency must be a multiple of the symbol rate. In addition, the known sampling theorem states that the sampling frequency must be at least twice as high as the maximum frequency of the sampled signal.
It is accordingly an object of the invention to provide a duplex data transmission method with QAM and a corresponding demodulator, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which has a further reduced complexity in the case of demodulation and which achieves a reduction in the sampling frequency.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for duplex data transmission with quadrature amplitude modulation via two conductors using time division multiplex and frequency division multiplex methods of two oppositely transporting channels, which comprises:
modulating a carrier signal with two digital data signals;
transmitting the modulated signal in a first direction in an upper channel having a relatively higher frequency band foxc2x1Bo/2 and in a second direction in a lower channel having a relatively lower frequency band fuxc2x1Bu/2, whereby the lower frequency band does not overlap the higher frequency band, and whereby each of the channels transports two digital signals, including a normal data signal and a quadrature data signal, at a respective symbol frequency fT;
sampling the modulated carrier signal received via the upper channel at a sampling frequency fAo and sampling the modulated carrier signal received via the lower channel at a sampling frequency fAu;
subsequently demodulating a pulse train resulting from the sampling step; and
subsequently low-pass filtering and sampling at the symbol frequency of the data signal for recovering the two digital data signals used for modulation at the respectively other end;
wherein the following frequencies are defined:
fo=center frequency of the upper channel;
Bo=bandwidth of the upper channel;
fu=center frequency of the lower channel;
Bu=bandwidth of the lower channel;
fT=symbol frequency of the digital data signal;
fAo=sampling frequency of a receiver-end sampling of the upper channel prior to demodulation;
fAu=sampling frequency of the receiver-end sampling of the lower channel prior to demodulation;
and wherein:
fo greater than fu, fAo greater than fT, fAu greater than fT;
fo/fu is a rational number K; and
fAo=2fo.
These relationships may be partly combined to require the ratio fo/fu to be a rational number K greater than 1.
By definition a rational number is a number which can be expressed as the quotient of two integers with a non-zero denominator.
In accordance with an added feature of the invention, a delay is introduced at the receiver end in the demodulation path of a respective one of the two data signals of at least one of the channels for phase-shifting the respective data signal with respect to a cyclic multiplier used for demodulation.
In accordance with an additional feature of the invention, the normal data signal and the quadrature data signal are brought into phase after demodulation by low-pass filtering with a do low-pass filter having a signal propagation time longer than a signal propagation time of a low-pass filter of another demodulation path by the same delay in the other of the two data signals.
In accordance with another feature of the invention, the rational number is an integer, and the delay may be set to 1/4fo.
In accordance with a combined feature of the invention, fAo=2xc2x7fo, the delay in the quadrature path of the upper channel amounts to 1/4fo, and the rational number is an integer.
In accordance with a further feature of the invention, during a demodulation of the upper channel, the sampling is split between two locations by sampling, prior to introducing the delay, at a sampling frequency (2fAo) twice the regular sampling frequency and, after introducing the delay, sampling each of the demodulation paths again at the regular sampling frequency fAo.
In accordance with again a further feature of the invention, the rational number is set to K=2.
In accordance with again another feature of the invention, the rational number (K) is set to be an odd integer, and during the demodulation of the lower channel, the sampling is split between two locations. This is done by sampling, prior to introducing the delay, at the doubled sampling frequency 2fAo and, after introducing the delay, sampling each of the demodulation paths again at the regular sampling frequency fAo.
In accordance with a concomitant feature of the invention, the delay is introduced in the quadrature demodulation path in the lower channel in an amount of K/4fo.
With the above and other objects in view there is provided, in accordance with the invention, a demodulator for use in the above outlined method for use in the upper channel or the lower channel, having a respective demodulation path for the normal signal and the quadrature signal. The demodulator comprises a first sampler for sampling at a sampling frequency, a multiplier connected to the first sampler for multiplying by a cyclic signal, a low-pass filter connected to the multiplier, a second sampler for sampling at the symbol frequency, and a delay element connected to an input of the first sampler.
In other words, the demodulator may be specially equipped for specific method implementations in that the delay element is placed upstream of the input of the first sampler.
By virtue of the fact that a rational ratio exists between the upper and lower carrier frequencies, an only finite number of values are obtained for the cosine and sine values of the lower carrier oscillations, and they can be stored in a read-only memory. The sampling frequency can be reduced to twice the center frequency of the respective upper channel, even though the sampling theorem is apparently violated in this case. This is possible because the spectral components of the upper channel which are superimposed on one another during sampling have fully symmetrical signal components with respect to the upper center carrier frequency owing to the modulation.
In the case of specific configurations, the situation might arise whereby during demodulation by multiplication by cyclic values, these values regularly correspond to the zero crossing. The sampled value would then be regularly multiplied by zero and the relevant data signal could not be recovered in this way. This problem is solved in that a delay by a time tv is introduced at the receiver end in the demodulation path of in each case one of the two data signals of at least one of the components, that is to say normal or quadrature components, for the purpose of phase-shifting it with respect to the cyclic multiplier used for demodulation. The multiplications are then carried out with values that are not equal to zero. The additionally introduced delay can be compensated for again in that the normal and quadrature data signals are brought in phase again after demodulation in that a low-pass filter whose signal propagation time is longer than the signal propagation time of the low-pass filter of the other demodulation path by the delay that has been introduced is used for that demodulation path of the channel which has not been allocated the delay.
Specific preferred configurations are produced if the rational number K is chosen to be an integer, where it is necessary in turn to decide whether the integer is even or odd; preferred values are K=2 or K=3. The delay that is to be introduced and is dependent on the frequencies chosen amounts for example to K times a quarter period of the carrier signal. There may be cases in which it is advantageous if during the demodulation of the upper or lower channel, the sampling is split between two locations in that, prior to the introduction of the delay, sampling is effected at a sampling frequency twice that of the regular sampling and, after the introduction of the delay, each of the demodulation paths is sampled again at the regular sampling frequency fAo.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for duplex data transmission with QAM and demodulator for use in this method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.